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Short Communication |
From Zentrum für Molekularbiologische Medizin,*
Universität zu Köln, Köln, and Abteilung
Neuropathologie,
Neurozentrum der
Albert-Ludwigs-Universität, Freiburg, Germany
| Abstract |
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is a candidate regulator of VEGF expression during
development. We therefore investigated expression of HRF/HIF-2
in
hemangioblastomas and found the overexpression of VEGF mRNA in stromal
cells to be highly correlated with elevated expression levels of
HRF/HIF-2
mRNA. This finding is suggestive for a role of HRF in
VEGF-dependent vascular growth in hemangioblastomas and could provide a
link between transcriptional activation of the VEGF gene and loss of
function of the VHL gene product. (Am J Pathol 1998,
153:2529)
| Introduction |
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-subunit of HIF-1 was cloned by our and
other groups and was denominated HRF/EPAS-1/HLF (which stands for
HIF-related factor/endothelial PAS protein-1/HIF-like
factor)13-15
and recently renamed HIF-2
.16
In the mouse embryo, this new factor is expressed by endothelial cells,
neurons, smooth muscle cells, and a variety of epithelia. Endothelial
expression of HRF could be correlated to the in vitro
transactivation of the promoter of the receptor tyrosine kinase
tie-2,15
which is essential for signal transduction during
embryonic angiogenesis.17,18
In epithelia, the level of
mRNA expression is highly correlated with the expression of VEGF. In
addition, HRF acts as a transactivator of VEGF promoter constructs
in vitro.14
Thus, HRF is a novel candidate
regulator of angiogenesis. To elucidate whether HRF could also be
involved in the high VEGF expression observed in stromal cells of
hemangioblastomas, we examined a series of cerebellar hemangioblastomas
for the expression of VEGF and HRF and compared these data with normal
brain and cerebellum. We found high expression levels of HRF mRNA in
VEGF-producing stromal cells and the surrounding capillaries. This
finding is suggestive for a role of this transcriptional regulator in
VEGF-dependent vascular growth in hemangioblastomas. | Materials and Methods |
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Tumor specimens were immediately snap-frozen after their removal and stored at -70°C until further use. Fourteen hemangioblastomas (from patients both with and without clinically confirmed VHL disease) and four glioblastomas were taken from the brain tumor bank of the Department of Neuropathology, Freiburg University Medical Center. For control purposes, normal cerebellum and normal cerebrum from two patients without neurological disease were obtained postmortem and snap-frozen in liquid nitrogen.
RNA Extraction and Northern Analysis
Total RNA was isolated using the guanidinium thiocyanate
method.19
Aliquots of 20 µg were separated on 1% agarose
gels containing 0.66 mol/L formaldehyde in 1x MOPS buffer (40 mmol/L
3-(N-morpholino)propanesulfonic acid, 10 mmol/L
sodium acetate, and 1 mmol/L EDTA (pH 7.2) and transferred to nylon
membrane (Duralon-UV, Stratagene, La Jolla, CA) by standard procedures
in 20x SSC (1x SSC is 150 mmol/L NaCl, 15 mmol/L sodium citrate).
Probes for the mouse HRF cDNA (nt 1688-2344), human VEGF cDNA (kindly
provided by Dr. H. Weich, Braunschweig), human HIF-1
cDNA (500-bp
HindIII fragment, kindly provided by Dr. H. Marti, Bad
Nauheim), and ß-actin were labeled with [32P]dCTP using
a random primer labeling kit (Stratagene). Hybridizations were carried
out as described previously.20
In Situ Hybridization
In situ hybridization was performed as described
previously.13
Single-stranded sense and antisense
[35S]-labeled RNA probes were generated from a
PstI-HindIII fragment of HRF subcloned in
pBluescript and a hVEGF121 pBluescript plasmid using T3 and T7
polymerases (Stratagene) and [35S]UTP. Sections were
hybridized with 2.5 x 104
cpm/µl probe overnight in
hybridization buffer (50% formamide, 10% dextran sulfate, 10 mmol/L
Tris/HCl, pH 7.5, 10 mmol/L sodium phosphate, pH 6.8, 5 mmol/L EDTA,
150 µg/ml Escherichia coli tRNA, 0.1 mmol/L UTP, 1 mmol/L
ADPßS, 10 mmol/L
S-ATP, 1 mmol/L dithiothreitol, 1 mmol/L
2-mercaptoethanol) at 42°C. After washes at high stringency (2x SSC,
50% formamide, 10 mmol/L 2-mercaptoethanol, 48°C) slides were coated
with Kodak NTB-2 emulsion and exposed for 2 to 4 weeks in the dark.
Slides were developed in Kodak D-19 developer and counterstained with
Giemsa's solution (Merck, Darmstadt, Germany).
Immunohistochemistry
To identify endothelial cells, sectiones were incubated with anti-PECAM/CD31 mouse monoclonal antibody (BioGenex, San Ramon, CA), and binding of the antibody was visualized using the Peroxidase LSAB Kit (Dako, Glostrup, Denmark), according to the manufacturer's instructions.
| Results and Discussion |
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mRNA did not differ significantly between
normal tissues and hemangioblastomas. In four glioblastomas that were
analyzed for comparison, no elevated levels of HRF could be
detected, although in three of these the VEGF mRNA was clearly
up-regulated. The level of HIF-1
mRNA message was essentially the
same in all tumors and normal tissues examined (Figure 2)
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is the most likely mediator of gene activation in response to
hypoxia.21
In vitro, the HIF-1
transcription
factor has been shown to transactivate the VEGF promoter via binding to
a hypoxia-responsive element.22
Several groups observed an
increase in the protein level of HIF-1
via stabilization of the
protein under hypoxic conditions whereas no induction of mRNA was
observed.12,23
In addition, all in situ data on
HIF-1
mRNA that are available so far do not reveal a correlation
between expression of HIF-1
mRNA and VEGF. In contrast, mRNA
expression of HRF, the close relative of HIF-1
, is correlated with
expression of VEGF mRNA in the lung and in other
organs.13,14
This also holds true for hemangioblastoma
stromal cells. On the other hand, no such striking correlation between
HRF and VEGF mRNA expression was observed in glioblastomas. Only a
slight elevation of HRF mRNA level was observed in the palisading cells
(data not shown), which is consistent with the data from Northern blot
hybridization. Hemangioblastoma, which may serve as a model for pathological angiogenesis in response to overexpression of VEGF, is the most frequent manifestation of VHL disease.1 It may be speculated that loss-of-function mutations of the VHL gene result in increased gene expression of HRF in stromal cells of hemangioblastomas. VHL has recently been shown to control transcriptional elongation by interaction with elongin B and C.24,25 One possibility, therefore, is that VHL controls transcriptional elongation of HRF in stromal cells. As the histogenesis of stromal cells is undefined, and no stromal cell line is available at present, this hypothesis requires additional evaluation. Whether this proposed dysregulation could hold true for other VHL-associated lesions, such as kidney and adrenal gland tumors, remains also to be examined. It has been shown that VHL mutations in certain cell lineages lead to an increased stability of VEGF mRNA.26,27 This mechanism is different from transcriptional activation of VEGF, which is expected to occur in response to HRF. Additional experiments on the molecular interaction of VHL, HRF, and the VEGF gene are necessary.
The increased expression of HRF in capillary endothelial cells located in close vicinity of VEGF-overexpressing stromal cells in contrast to more distant endothelial cells may result from activation of endothelium by VEGF or other stromal-cell-derived factors. As HRF was found to be a transactivator of tie-2 promoter in vitro, it may be assumed that this transcription factor is involved in the transcriptional control of the tie genes, which are expressed in capillary endothelial cells in hemangioblastomas28 (our own unpublished observations). Thus, it may be speculated that VEGF receptors that are up-regulated in hemangioblastoma capillaries2 are also under the control of HRF.
In conclusion, our data show for the first time a correlation of HRF expression in a human tumor with overexpression of VEGF. The fact that this particular tumor is highly vascular underlines the central role this novel transcription factor may have for the regulation of vascular growth and differentiation.
| Acknowledgements |
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| Footnotes |
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Supported by grant C4 from the Center for Clinical Research I, Freiburg University Medical School, and by grant PI158/3-1 from the Deutsche Forschungsgemeinschaft to K. H. Plate.
Accepted for publication April 7, 1998.
| References |
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