p53 suppresses the self-renewal of adult neural stem cells

There is increasing evidence that tumors are heterogeneous and that a subset of cells act as cancer stem cells. Several proto-oncogenes and tumor suppressors control key aspects of stem cell function, suggesting that similar mechanisms control normal and cancer stem cell properties. We show here that the prototypical tumor suppressor p53, which plays an important role in brain tumor initiation and growth, is expressed in the neural stem cell lineage in the adult brain. p53 negatively regulates proliferation and survival, and thereby self-renewal, of neural stem cells. Analysis of the neural stem cell transcriptome identified the dysregulation of several cell cycle regulators in the absence of p53, most notably a pronounced downregulation of p21 expression. These data implicate p53 as a suppressor of tissue and cancer stem cell self-renewal.

A PDGFRA promoter polymorphism, which disrupts the binding of ZNF148, is associated with primitive neuroectodermal tumours and ependymomas

BACKGROUND

Platelet derived growth factor receptor alpha (PDGFRalpha) expression is typical for a variety of brain tumours, while in normal adult brain PDGFRalpha expression is limited to a small number of neural progenitor cells. The molecular mechanisms responsible for the PDGFRalpha expression in tumours are not known, but in the absence of amplification, changes in transcriptional regulation might be an important factor in this process.

METHODS AND RESULTS

We have investigated the link between single nucleotide polymorphisms (SNPs) within the PDGFRalpha gene promoter and the occurrence of brain tumours (medulloblastomas, supratentorial primitive neuroectodermal tumours (PNETs), ependymal tumours, astrocytomas, oligodendrogliomas, and mixed gliomas). These SNPs give rise to five different promoter haplotypes named H1 and H2alpha-delta. It is apparent from the haplotype frequency distribution that both PNET (10-fold) and ependymoma (6.5-fold) patient groups display a significant over-representation of the H2delta haplotype. The precise functional role in PDGFRalpha gene transcription for the H2delta haplotype is not known yet, but we can show that the H2delta haplotype specifically disrupts binding of the transcription factor ZNF148 as compared to the other promoter haplotypes.

CONCLUSIONS

The specific over-representation of the H2delta haplotype in both patients with PNETs and ependymomas suggests a functional role for the ZNF148/PDGFRalpha pathway in the pathogenesis of these tumours.

p53 must be competent for transcriptional regulation to suppress tumor formation

In vitro studies suggest that effective tumor suppression by p53 requires multiple domains to execute transcription-dependent and transcription-independent functions. We generated a mutant p53 allele in mice, p53(W25QL26S) (p53(QS)), containing an inactive transactivation domain to evaluate the importance of transactivation for p53-mediated tumor suppression. Recently, we discovered that the allele also contains a valine substitution for alanine at codon 135, which borders the DNA-binding domain. We found that p53(QSval135) bound to chromatin albeit less well than p53(QSala135), but both were equally deficient in transcriptional regulation, apoptosis induction in mouse embryo fibroblasts (MEFs), and suppression of tumor formation by E1A, Ha-Ras transformed MEFs. p53(QSval135) mice and p53-null mice exhibited identical tumor development kinetics and spectra in spontaneous and oncogene-initiated tumorigenicity assays, when tested in a homo- and heterozygous configuration. The p53(QSval135) allele did not have dominant negative functions and behaved as a null allele. Taken together, these data indicate that effective tumor suppression requires the transcriptional regulation function of p53, and they suggest that transactivation independent functions of p53 are unlikely to contribute significantly to tumor suppression in vivo.

Molecular cloning and characterization of two novel truncated isoforms of human Na+/Ca2+ exchanger 3, expressed in fetal brain

The human gene encoding the Na+/Ca2+ exchanger family member 3 (NCX3) undergoes extensive alternative splicing, with four variants previously identified. In this study, we report two novel alternative transcripts encoding two N-terminally truncated NCX3 proteins specifically expressed in human fetal brain. The identified transcripts, designated NCX3-tN.1 and NCX3-tN.2, are approximately 2.8 kb and 2.9 kb, respectively. The open reading frames (ORFs) are predicted to encode separately a 284 and a 298 amino acid (aa) polypeptide. Sequence analysis and bioinformatics reveal that NCX3-tN.1 and NCX3-tN.2 are the result of alternative splicing of the NCX3 gene. They have their own potential start codons and unique 5' untranslated regions (UTRs) that are different from those of the known NCX3 variants. The variants include a part of intron 2 of the original gene organization as their first exon (exon "a") at the 5' end of the novel transcripts. NCX3-tN.2 consists of six exons including exon "a" and exons 4, 6, 7, 8 and 9 of NCX3, while NCX3-tN.1 lacks exon 4, but is otherwise similar to NCX3-tN.2. Expression studies show that both variants can be translated into protein and NCX3-tN.1 seems more efficiently translated. Based on their structural features, NCX3-tN.1 and NCX3-tN.2 proteins are potentially involved in regulation of Na+/Ca2+ homeostasis.

Gli1 is not required for Pdgfrα expression during mouse embryonic development

Pdgfra is expressed in the mesenchyme of multiple organs during embryonic development and Pdgfralpha is involved in cell proliferation, differentiation, migration, and apoptosis in many tissues. A fine-tuned regulation of gene transcription is required to achieve these effects. To investigate if the Shh signaling pathway is involved in the tightly regulated Pdgfra expression during embryogenesis, we systematically compared Gli1 and Pdgfralpha mRNA expression patterns in vivo from mouse embryonic day 9.5 to 14.5. We found that an initial partly overlapping expression of Gli1 and Pdgfralpha in the mesenchyme of foregut and somites was changed to different expression patterns when the mesenchyme differentiated into specialized structures such as intestinal villi and chondrocytes. Gli1 and Pdgfra were also expressed differently in the developing lung, heart, central nervous system, skin, tooth, and eye. Importantly, neither Pdgfralpha mRNA patterns nor levels were altered in Ihh mutant embryos although Gli1 and Ptc mRNA levels were dramatically reduced. Our results demonstrate that Gli1 is not required to induce Pdgfra expression during embryonic bone development, and are consistent with previous findings that Pdgfralpha and Hh pathways serve different functions in, e.g., bone, gut, and lung development. However, we cannot exclude the possibility that Glis can have more complex regulatory effects on Pdgfra gene activity, nor can we exclude such effects in pathological conditions.

Forced expression of platelet-derived growth factor B in the mouse cerebellar primordium changes cell migration during midline fusion and causes cerebellar ectopia

The platelet-derived growth factor (PDGF) and receptors are expressed in the developing central nervous system and in brain tumors. To investigate the role of PDGF during normal cerebellar development, we created transgenic mice where PDGF-B was introduced into the endogenous Engrailed1 locus (En1). These mice expressed PDGF-B in all types of cells that constitute the developing cerebellum, with localized high expression in the ventral midline of the cerebellar anlage. This affected cell migration in the midline during fusion of the cerebellar anlage and caused misplacement of midline structures. PDGFR-alpha- and laminin alpha1-positive meningeal cells migrated inwards, attracted by the ectopic transgene expression in the ventral neuroepithelium. Other cells followed the meningeal cells and in the adult mouse, cells from all cortical cell layers were found misplaced in the midline. Moreover, the transgene caused an enhancement of capillary vessels. The findings indicate that normal PDGF signaling is important for proper neural tube fusion. It also illustrates that meningeal structures can influence the process.

C/EBP is an essential component of PDGFRA transcription in MG-63 cells

Interleukin-1beta (IL-1beta) is a potent inhibitor of platelet-derived growth factor alpha receptor (PDGFRalpha) expression in MG-63 cells. Its effect is mediated at the transcriptional level, but the transcription factors involved in this process are unknown. In the current study, we found that IL-1beta could inhibit the PDGFRalpha gene promoter activity, and this effect was strongly correlated with increased binding of CCAAT/enhancer-binding protein (C/EBP) to the responsive promoter region. In addition, forced expression of C/EBPbeta could mimic the IL-1beta effect on the promoter activity, but subsequent mutation analysis of the C/EBP binding sites indicated that direct C/EBP binding to the promoter is not required for the IL-1beta response. However, our data clearly demonstrated that the C/EBP binding site at position-162 relative to the transcriptional start site is essential for high basal level PDGFRalpha promoter activity.

Complementary effects of platelet-derived growth factor autocrine stimulation and p53 or Ink4a-Arf deletion in a mouse glioma model

INK4a-ARF and p53 inactivation are common but rarely concurrent findings in glioblastoma multiforme. Here we demonstrate that experimental deletion of either tumor suppressor gene cooperates with retrovirally expressed platelet-derived growth factor (PDGF)-B regarding both tumor latency and frequency in a mouse brain tumor model. We find indications of PTEN down-regulation and increased Akt phosphorylation in both types of null tumors (although more prominent in p53-/- tumors) suggesting a possible mechanism for this synergism. This is the first time that the cooperative tumorigenic effects of PDGF-B stimulation and p53 loss of function are demonstrated in an in vivo model, establishing a functional link between two common molecular changes of human secondary glioblastoma multiforme.

Oligodendrocyte precursor hypercellularity and abnormal retina development in mice overexpressing PDGF-B in myelinating tracts

Platelet-derived growth factor (PDGF) influences the generation of neurons and glia during embryogenesis and in early postnatal life. In an attempt to determine the consequences of an overexpression of PDGF-B during the first weeks of life, we targeted transgenic expression of a human PDGF-B cDNA to myelinating tracts using the promoter region of the myelin basic protein (MBP) gene. Transgenic mRNA and protein were expressed in the brain and the expression profile of the human PDGF-B during early postnatal development closely paralleled that of the endogenous mouse MBP gene. The gross morphological appearance of transgenic brains was normal but at the cellular level several phenotypic alterations could be identified. In white matter tracts such as the corpus callosum and cerebellar medulla, there was a marked hypercellularity. The number of oligodendrocyte precursors was increased and astrocytes were more abundant. In adult mice carrying the MBP-PDGF-B transgene, however, myelination appeared normal and the amount of oligodendrocytes was similar to that of control littermates. In addition to the phenotypic alterations in the brain, investigation of eye structure revealed a striking disorganization of retinal architecture. The retina was folded with cells collected in papillar or follicular-like structures. Retinal whole mount preparations after India ink perfusion revealed capillary disorganization with large-caliber vessels supporting only a few fine branches. Our observations strengthen the notion that PDGF is an important effector molecule in postnatal CNS development.

Platelet-derived growth factor-B and -C and active alpha-receptors in medulloblastoma cells

The malignant childhood brain tumor medulloblastoma belongs to the group of primitive neuroectodermal tumours (PNETs). Medulloblastomas are thought to arise from remnants of the transient external germinal layer in the cerebellum. Proliferation, differentiation, and motility of cells in the central nervous system are regulated by growth factors, e.g., platelet-derived growth factor (PDGF). Recently, it was shown that higher level of PDGF alpha-receptor expression is characteristic of metastatic medulloblastomas. We have investigated five medulloblastoma/PNET cell lines and found that the PDGF alpha-receptor is actively signalling in most of them, an activity most likely driven by endogenously produced PDGF-C. PDGF-C is normally present in cells of the developing external germinal layer and our results are consistent with the idea that medulloblastomas are derived from such cells undergoing early neuronal differentiation. Moreover, the expression of PDGF and its receptors was associated with neuronal characteristics, but not with high levels of c-myc expression in the medullablastoma cells.

Prognostic value of platelet derived growth factor alpha receptor expression in grade 2 astrocytomas and oligoastrocytomas

OBJECTIVE

To determine whether the expression of platelet derived growth factor alpha receptor (PDGFRalpha) in low grade astrocytomas and oligoastrocytomas is associated with survival.

METHODS

Formalin fixed and paraffin embedded tumour samples of 40 consecutive patients with supratentorial diffuse astrocytomas and oligoastrocytomas of WHO grade 2, resected between 1986 and 1993, were used for immunohistochemical staining. The fraction of tumour cells expressing PDGFRalpha protein was quantified and entered into univariate and multivariate survival analyses. Changes in PDGFalpha expression over time were analysed in seven patients in whom reoperations had been performed.

RESULTS

Patients with a relatively high fraction of PDGFRalpha expressing cells had a more favourable outcome in both univariate (p = 0.04) and multivariate analyses (p = 0.02). Expression of PDGFRalpha was greater in oligoastrocytomas than in astrocytomas (p = 0.05). In four reoperated patients with histologically confirmed malignant transformation, there was a marked decrease in the number of cells expressing the receptor.

CONCLUSIONS

There is an association between high PDGFRalpha expression and long survival time in patients with grade 2 astrocytomas and oligoastrocytomas. The findings suggest that expression of the receptor may be a useful prognostic marker in such patients.

Clonality analysis of synchronous lesions of cervical carcinoma based on X chromosome inactivation polymorphism, human papillomavirus type 16 genome mutations, and loss of heterozygosity

One of the most common forms of carcinoma in women, cervical invasive squamous cell carcinoma (CIC), often coexists with multiple lesions of cervical intraepithelial neoplasia (CIN). CIC and CIN show heterogeneity with respect to both histopathology and biology. To understand the causes, origin, and model of progression of cervical carcinoma, we assessed the clonality of a case with multiple synchronous lesions by analyzing X chromosome inactivation polymorphism, human papillomavirus type 16 (HPV16) sequence variation/mutations, and loss of heterozygosity (LOH). Microdissection was performed on 24 samples from this case, representing the entire lesional situation. The combination of different X chromosome inactivation patterns, two HPV16 point mutations, and LOH at three genomic microsatellite loci, led to the identification of five different "monoclonal" lesions (CIN II, CIN III, and invasive carcinoma nests) and five different "polyclonal" areas (CIN II and normal squamous epithelium). This finding indicated that CIC can originate from multiple precursor cells, from which some clones might progress via multiple steps, namely via CIN II and CIN III, whereas others might develop independently and possibly directly from the carcinoma precursor cells. Our results also supported the view that HPV16 as a "field factor" causes cervical carcinoma, which is probably promoted by the loss of chromosomal material as indicated by the LOH.

TGF-alpha-driven tumor growth is inhibited by an EGF receptor tyrosine kinase inhibitor

The simultaneous presence of the EGFR and its ligand TGF-alpha in human tumor tissues suggests that autocrine TGF-alpha stimulation drives tumor growth. Here we show that autocrine TGF-alpha stimulation does cause increased tumor growth in vivo, an effect that was proven to be mediated via EGFR activation, and that this TGF-alpha/EGFR autocrine loop was accessible to an EGFR specific tyrosine kinase inhibitor. Clones of the EGFR expressing glioma cell line U-1242 MG were transfected with TGF-alpha cDNA using a tetracycline-inhibitory system for gene expression. TGF-alpha expression was inhibited by the presence of tetracycline, and subcutaneous tumors forming from cell lines injected into nude mice could be inhibited by feeding mice tetracycline. We confirmed that TGF-alpha mRNA and protein were present in these tumors and that, subsequently, the endogenous EGFR was activated. Tumor growth could be inhibited by an EGFR specific tyrosine kinase inhibitor of the type 4-(3-chloroanilino)-6,7-dimethoxy-quinazoline, administered daily by intraperitoneal injection, thereby interrupting the autocrine loop.

Oncogene lineages of human papillomavirus type 16 E6, E7 and E5 in preinvasive and invasive cervical squamous cell carcinoma

Human papillomavirus (HPV)16 accounts for about 60% of the HPV infections in invasive cervical cancer (ICC). There are many sequence variations within HPV16, some of which have been associated with different biological properties, although no definite correlations have yet been established. However, the definition 'variant' has been a source of confusion in research and diagnosis, since it is based on all sequence deviations from a randomly selected prototype. This study has sequenced the HPV16 oncogenes E6, E7 and E5 from 61 Swedish cases with cervical intraepithelial neoplasia grade III (CIN III) or ICC. Clustering the sequence variations at the three common sites of variation (nucleotide 350 in E6, which has previously been associated with the progression from CIN III to ICC, and nucleotides 3979 and 4042 in E5) resulted in the distinction of three major oncogene lineages encompassing more than 95% of the cases, and two minor oncogene lineages. Simple comparison of the distribution of the individual variations or oncogene lineages between CIN III and ICC showed no significant difference, but the number of variations in addition to the three common ones was significantly higher in ICC. This novel classification scheme, based on the variations in the E6, E7 and E5 region, is considered to be a major improvement over the classical 'prototype-variant' classification, and can help to clarify the interpretation of HPV sequence data in relation to the progression of cervical cancer.

A 1.8kb GFAP-promoter fragment is active in specific regions of the embryonic CNS

The intermediate filament glial fibrillary acidic protein (GFAP) constitutes the major cytoskeletal protein in astrocytes (J. Neuroimmunol. 8 (1985) 203) and is traditionally referred to as a specific marker for astrocytes. To identify early glial precursors, we created GFAPpromoter-lacZ transgenic mice, using a 1.8kb 5' fragment of human GFAP. The expression of the transgene was first detected in the neuroepithelium at embryonic day 9.5. It was further found in the ventricular zone of the developing telencephalon, in the cerebellar primordium, trigeminal ganglia, and radial glia. Later, scattered beta-gal+ cells were seen in pons, brain stem and glia limitans. The results indicate that GFAP activity is regulated in a region-specific manner during central nervous system (CNS) development and that the gene is turned on in different cell types independently.

Recovering DNA and optimizing PCR conditions from microdissected formalin-fixed and paraffin-embedded materials

Microdissection is a powerful technique in molecular pathology and genetic investigations. To detect genetic alterations such as gene mutation or deletion from tumor specimen, the purity of target cells is extremely critical. Unwanted cell contamination will dramatically dilute the detectable level of the abnormality. The major obstacle in clinical research is to obtain sufficient and qualified DNA from a small amount of formalin-fixed and paraffin-embedded materials. We have successfully modified our previous protocols and overcome the difficulties of recovery of DNA. After these modifications, almost every single formalin-fixed and paraffin-embedded specimen has been successfully amplified in the required DNA region.

Platelet-derived growth factor receptor-alpha in ventricular zone cells and in developing neurons

Cells in the early neuroepithelium differentiate and give rise to all cells in the central nervous system (CNS). The ways from a multipotent CNS stem cell to specialized neurons and glia are not fully understood. Using immunohistochemistry we found that neuroepithelial cells express the platelet-derived growth factor receptor-alpha (PDGFR-alpha) in the neural plate at embryonic day 8.5 and onwards in the neural tube. The protein was polarized to ventricular endfeet. Furthermore, PDGFR-alpha expression was localized to cells undergoing early neuronal development. We also found PDGFR-alpha expression in developing granule cells in the postnatal cerebellum, in Purkinje cells in the adult cerebellum and on processes of developing dorsal root ganglion cells. Previous reports mainly describe PDGFR-alpha expression in oligodendrocyte precursors and glial cells. We believe, in line with a few previous reports, that the PDGFR-alpha in addition marks a pool of undifferentiated cells, which are able to differentiate into neurons.

HPV16 E6 gene variations in invasive cervical squamous cell carcinoma and cancer in situ from Russian patients

HPV16 is frequently seen in invasive cervical cancer (ICC) and cervical intraepithelial neoplasia (CIN). Its E6 gene has frequent sequence variations. Although some E6 variants have been reported to have different biochemical or biological properties, they do not show geographical identity. Moreover, the definition of 'variant' has been a source of confusion because it has been based on all departures from the 'prototype' once isolated randomly from an ICC case. We amplified the HPV16 E6 gene by PCR from fresh-frozen tissue of 104 cases of ICC and CIN from Russian patients and sequenced it in positive cases. We found that 32 of 55 (58.2%) ICC cases and 18 of 49 (36.7%) CIN cases were HPV 16-positive and we could identify 3 groups of E6 variants: group A was characterized by G at nt 350 where group B had T, and group M was a heterogeneous mixture of unique E6 variants; no significant difference existed in the distribution of the different groups between ICC and CIN; the clinically malignant (as defined by FIGO stage) order between the groups was M > A > B in ICC; in the cases with a single HPV16 E6 sequence, coexisting ICC, CIN and normal epithelium in the same patient shared the E6 variant; and 4 cases of ICC had double/multiple E6 variants. The results did not show any importance of E6 variants for ICC progression in Russian women. The results also indicated that the original HPV16 variant persisted during ICC progression, and that at a low frequency, double infections and/or mutation of variants might occur.

A human YAC transgene rescues craniofacial and neural tube development in PDGFRalpha knockout mice and uncovers a role for PDGFRalpha in prenatal lung growth

The platelet-derived growth factor alpha-receptor (PDGFRalpha) plays a vital role in the development of vertebrate embryos, since mice lacking PDGFRalpha die in mid-gestation. PDGFRalpha is expressed in several types of migratory progenitor cells in the embryo including cranial neural crest cells, lung smooth muscle progenitors and oligodendrocyte progenitors. To study PDGFRalpha gene regulation and function during development, we generated transgenic mice by pronuclear injection of a 380 kb yeast artificial chromosome (YAC) containing the human PDGFRalpha gene. The YAC transgene was expressed in neural crest cells, rescued the profound craniofacial abnormalities and spina bifida observed in PDGFRalpha knockout mice and prolonged survival until birth. The ultimate cause of death was respiratory failure due to a defect in lung growth, stemming from failure of the transgene to be expressed correctly in lung smooth muscle progenitors. However, the YAC transgene was expressed faithfully in oligodendrocyte progenitors, which was not previously observed with plasmid-based transgenes containing only upstream PDGFRalpha control sequences. Our data illustrate the complexity of PDGFRalpha genetic control, provide clues to the location of critical regulatory elements and reveal a requirement for PDGF signalling in prenatal lung growth, which is distinct from the known requirement in postnatal alveogenesis. In addition, we found that the YAC transgene did not prolong survival of Patch mutant mice, indicating that genetic defects outside the PDGFRalpha locus contribute to the early embryonic lethality of Patch mice.

Effects of inducible glial fibrillary acidic protein on glioma cell motility and proliferation

We have studied the effect of induced glial fibrillary acidic protein (GFAP) on motility, cell morphology, and proliferation of two originally GFAP-negative human glioma cell lines. Glioma cell lines U-1242 MG and U-251 MG sp subclone 3A were transfected with a vector system that allows for an inducible GFAP expression. This experimental system creates an "on/off" situation in which GFAP expression is suppressed by tetracycline. Inducible expression of GFAP in the absence of tetracycline was confirmed by immunofluorescence staining and Northern and Western blotting. The study showed that forced GFAP expression resulted in an inhibition of cell motility measured as the phagokinetic track area of individual cells seeded sparsely on a surface covered with gold particles. It also resulted in a change in cell morphology, with extended cell processes, and it was associated with a low fraction of cells in S-phase. We conclude that the down-regulation of GFAP expression that is often seen in gliomas in vivo may be an important parameter of tumor progression related mainly to the motile and thereby invasive properties of malignant glioma cells.

Modulation of phenotype and induction of irregular vessels accompany high tumorigenic potential of clonal human glioma cells xenografted to nude-rat brain

Three phenotypically different clonal human glioma cell lines were injected stereotactically into nude-rat brains, to determine their individual growth potential and to establish an in vivo system in which different therapeutic modalities could be tested. As assessed by serial sectioning, microscopic evaluation, and computer analysis, the mean approximate tumour volume after 3-7 weeks in vivo was 0.42 mm(3) for U-343 MG, 2.6 mm(3) for U-343 MGa Cl2:6, and 50.3 mm(3) for U-343 MGa 31L. When compared with the initial injected cell volume, only U-343 MGa 31L had increased in size, U-343 MGa Cl2:6 remained approximately the same but showed a certain proliferative potential, and U-343 MG regressed. Thus, only U-343 MGa 31L cells had high tumorigenic potential, invaded and replaced brain tissue in every direction, while U-343 MGa Cl2:6 cells grew in sheet-like tumour extensions along white-matter nerve-fibre tracts, in this respect mimicking foetal astrocytes. The tumorigenic potential of the U-343 MGa 31L cell clone was associated with a variable phenotype, as observed when the in vivo and in vitro characteristics were compared. The in vivo phenotype was characterized by the loss of GFAP immunoreactivity, the gain of heterogeneously distributed cellular tenascin, fibronectin, and laminin, but absence of extracellularly deposited material, and by the formation of irregular vessels. It appears that the intrinsic capacity of glioma cells to adapt to in vivo conditions is decisive for their tumorigenicity in the brain, rather than any single phenotypic property in itself. Moreover, the 2 glioma cell clones best suited for in vitro growth were no longer tumorigenic.

Dependence of autocrine growth factor stimulation in platelet-derived growth factor-B-induced mouse brain tumor cells

In human gliomas, platelet-derived growth factor (PDGF) ligand and receptor mRNA are often co-expressed, which suggests the presence of an autocrine loop. To further investigate the significance of PDGF stimulation in brain tumors, we used a previously developed mouse tumor model, in which malignant brain tumors of neuroepithelial origin were induced by injecting a murine retrovirus containing the human PDGF B-chain gene into the brains of neonatal mice. In the present investigation, we have characterized a cell line established from such an experimentally induced tumor in an INK4a-/- mouse. Cultured tumor cells expressed nestin and NG2 chondroitin sulfate proteoglycan and are thus most likely derived from an oligodendrocyte precursor cell. Tumor cells produced PDGF-B protein and displayed constitutively activated PDGF alpha receptors. Autocrine receptor activation could be blocked with the specific PDGF receptor tyrosine kinase inhibitor CGP 57148B, which led to almost complete inhibition of cell proliferation, which was much less affected by a PDGF B-chain aptamer that inhibits binding of PDGF-B to PDGF receptors and is unlikely to be able to pass through the plasma membrane. Our results imply an important role for PDGF autocrine stimulation in both initiation and progression of a subtype of gliomas.

PDGF B mRNA variants in human tumors with similarity to the v-sis oncogene: expression of cellular PDGF B protein is associated with exon 1 divergence, but not with a 3'UTR splice variant

While platelet-derived growth factor, PDGF, is not regularly expressed in mesenchymal tissues, PDGF B mRNA is often found in tumors derived from these tissues. PDGF B protein is also present, but the protein levels in individual tumors do not appear to correlate well with those of the mRNA. PDGF B is homologous to the v-sis oncogene of simian sarcoma virus (SSV), and certain deletions confined to 3; and 5; non-coding sequences of PDGF B mRNA are consistently found in tumors induced by SSV and by a PDGF B retrovirus. Part of exon 1, including a non-coding GC-rich regulatory domain and the signal sequence as well as a 149 nucleotide long AU-rich stretch in the 3; non-coding region, are often lost. We hoped that this information could provide a clue to defective regulatory mechanisms present in human tumors and have searched for such mRNA variants in human sarcoma cell lines and soft tissue tumors. We identified a splice variant of PDGF B mRNA that, similar to v-sis, lacks the 149 nucleotide stretch and introduces an anomalous splice point between exons 6 and 7. This weakly abundant mRNA variant was co-expressed with the regular PDGF B mRNA, but its presence failed to show any association with increased levels of immunohistochemically detectable PDGF B protein. Instead, the highest levels of cellular PDGF B protein were found in samples with mRNAs showing exon 1 divergence. The changes in the 5; end of the mRNA were not accompanied by any genomic aberrations.