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The tumor suppressor protein VHL targets hypoxia-inducible factors for oxygen-dependent proteolysis

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ac.uk)
The tumour suppressor
protein VHL targets
hypoxia-inducible factors for
oxygen-dependent proteolysis
Patrick H. Maxwell*, Michael S. Wiesener*,
Gin-Wen Chang*, Steven C. Clifford, Emma C. Vaux,
Matthew E. Cockman, Charles C. Wykoff,
Christopher W. Pugh, Eamonn R. Maher
& Peter J. Ratcliffe*
*Wellcome Trust Centrefor Human Genetics, WindmillRoad, OxfordOX3 7BN,UK
Section of Medical and Molecular Genetics, Department of Paediatrics and Child
Health, University of Birmingham, Birmingham B15 2TT, UK
Institute of Molecular Medicine, John Radcliffe Hospital, Oxford OX3 9DS, UK
.........................................................................................................................
Hypoxia-inducible factor-1 (HIF-1) has a key role in cellular
responses to hypoxia, including the regulation of genes involved
in energy metabolism, angiogenesis and apoptosis1–4. The a
subunits of HIF are rapidly degraded by the proteasome under
normal conditions, but are stabilized by hypoxia5. Cobaltous ions
or iron chelators mimic hypoxia, indicating that the stimuli may
interact through effects on a ferroprotein oxygen sensor6,7. Here
we demonstrate a critical role for the von Hippel-Lindau (VHL)
tumour suppressor gene product pVHL in HIF-1 regulation. In
VHL-defective cells, HIF a-subunits are constitutively stabilized
and HIF-1 is activated. Re-expression of pVHL restored oxygen-
dependent instability. pVHL and HIF a-subunits co-immunopre-
cipitate, and pVHL is present in the hypoxic HIF-1 DNA-binding
complex. In cells exposed to iron chelation or cobaltous ions,
HIF-1 is dissociated from pVHL. These findings indicate that the
interaction between HIF-1 and pVHL is iron dependent, and
that it is necessary for the oxygen-dependent degradation of
HIF a-subunits. Thus, constitutive HIF-1 activation may underlie
the angiogenic phenotype of VHL-associated tumours. The
pVHL/HIF-1 interaction provides a new focus for understanding
cellular oxygen sensing.
Enhanced glucose metabolism and angiogenesis are classical
features of cancer8,9, involving upregulation of genes that are
normally induced by hypoxia. In addition to stimulation by the
hypoxic microenvironment10, genetic alterations contribute to these
effects8,9. A striking example is von Hippel-Lindau (VHL) disease, a
hereditary human cancer syndrome predisposing sufferers to highly
angiogenic tumours11. Constitutive upregulation of hypoxically
inducible messenger RNAs encoding vascular endothelial growth
factor (VEGF) and glucose transporter 1 (GLUT-1) in these tumour
cells is correctable by re-expression of pVHL. A post-transcriptional
mechanism has been proposed12,13. We studied the involvement of
pVHL in oxygen-regulated gene expression using ribonuclease
protection analysis of two VHL-deficient renal carcinoma lines,
RCC4 and 786-O. Eleven genes encoding products involved in
glucose transport, glycolysis, high-energy phosphate metabolism
and angiogenesis were examined; nine are induced by hypoxia in
other mammalian cells and two (LDH-B and PFK-M) are repressed
by hypoxia. None of these responses was seen in the VHL-defective
cell lines. Responses to hypoxia were restored by stable transfection
of a wild-type VHL gene, with effects ranging from a rather modest
action of hypoxia (PFK-L and LDH-B) to substantial regulation
(Fig. 1 shows results for RCC4 cells; similar effects were seen in 786-
O cells, data not shown). These results indicate that the previously
described upregulation of hypoxia-inducible mRNAs in VHL-
defective cells12,13 extends to a broad range of oxygen-regulated
genes and involves a constitutive ‘hypoxia pattern’ for both posi-
tively and negatively regulated genes.
As a number of these genes (VEGF, GLUT-1, AK-3, ALD-A,
PGK-1, PFK-L and LDH-A) contain hypoxia-response elements
(HREs) which bind HIF-1 and/or show altered expression in cells
lacking HIF-1 (refs 2, 14 and references therein), this survey of
expression in VHL-defective cells prompted us to look for effects of
pVHL on HIF-1 and HRE function. RCC4 cells were co-transfected
with reporter plasmids which did or did not contain HREs from the
mouse phosphoglycerate-kinase-1 or erythropoietin (Epo) enhan-
cers, and either the VHL-expression plasmid pcDNA3-VHL or an
empty vector. pVHL suppressed HRE activity in normoxic cells and
restored induction by hypoxia (Fig. 2a). Similar results were
obtained by sequential stable transfection of RCC4 cells with an
HRE reporter followed by pcDNA3-VHL (data not shown). HIF-1
itself was examined by electrophoretic mobility shift assay (EMSA),
which showed a constitutive HIF-1 DNA-binding species in VHL-
deficient RCC4 cells, with restoration of the normal hypoxia-
inducible pattern in RCC4 cells stably transfected with pcDNA3-
VHL (RCC4/VHL) (Fig. 2b). In other cells, HIF-1 activation by
hypoxia involves a large increase in HIF-1aabundance from low
basal levels in normoxia1,15. Western blotting of whole-cell extracts
showed that RCC4 cells express constitutively high levels of both
HIF-1aand a related molecule, HIF-2a(also termed EPAS-1, HRF,
HLF and MOP2) which is normally regulated in a similar way16
(Fig. 2c). Constitutively high levels of these proteins were found in
eight other VHL-defective cell lines, in contrast to the renal
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carcinoma line Caki-1 (which expresses pVHL normally17) and a
wide range of previously reported cell lines (Fig. 2d and Supple-
mentary Information S2). Certain VHL-defective cells (for example,
786-O, KTCL140) expressed HIF-2aat a high constitutive level but
did not express detectable HIF-1a(Fig. 2c, d and Supplementary
Information S2). Examination of stable transfectants of RCC4 and
786-O cells demonstrated that expression of the wild-type, but not
truncated, VHL gene restored regulation of HIF a-subunits by
oxygen without affecting the levels of mRNA encoding either
subunit (data not shown).
To investigate the role of pVHL in HIF-1 regulation we tested for
interactions between HIF a-subunits and pVHL using a combina-
tion of hypoxia and/or proteasomal blockade to induce HIF a-
subunits. Anti-pVHL immunoprecipitates of extracts from protea-
somally blocked RCC4/VHL cells, but not RCC4 cells, contained
both HIF-1aand HIF-2a(Fig. 3a). Similar results were obtained
with hypoxia in the absence of proteasomal blockade. In the inverse
reactions, immunoprecipitating antibodies to HIF-2aor HIF-1a
co-precipitated pVHL, although a smaller proportion of the total
was captured (Fig. 3b). Anti-pVHL immunoprecipitations also
demonstrated the interaction in HeLa cells, which express pVHL
normally (Fig. 3c). We next tested whether pVHL is incorporated in
the HIF-1 DNA-binding complex. Addition of antibodies raised
against pVHL (but not control antibodies) to nuclear extract from
RCC4/VHL cells and HeLa cells produced a change in mobility,
which was not observed with VHL-defective RCC4 cells (Fig. 3d).
HIF-1 migrated as two species. Only the slower-mobility HIF-1
species was shifted by anti-pVHL, whereas both species were shifted
Figure 1 Effect of pVHL on oxygen-regulated gene expression. mRNA analysis of
RCC4 cells and stable transfectant expressing pVHL (RCC4/VHL). N, normoxia;
H, hypoxia (1% O
2
, 16 h). VEGF, vascular endoth elial growth factor; GLUT-1,
glucose transporter 1; AK-3, adenylate kinase 3; TGF-b1, transforming growth
factor-b1; ALD-A, aldolase A; PGK-1, phosphoglyceratekinase 1; PFK, phos-
phofructokinase; LDH, lactate dehydrogenase. U6 small nuclear (sn) RNA was
used as an internal control. Also illustrated are two genes not influenced by VHL
status or hypoxia; nuclear respiratory factor 1 (NRF-1) and b-actin. Amount of RNA
analysed is detailed in Table S1 of the Supplementary Information.
Figure 2 Effect of pVHL on HIF-1 and HRE activity. a, Representative transient
transfections of RCC4 cells with VHL expression vector (+) or empty vector, and
luciferase reporter genes containing no HRE, or HREs from the PGK-1 or
erythropoietin (Epo) genes linked to SV40 or TK promoters. N, normoxia; H,
hypoxia (0.1% O
2
, 24 h —results were similar with 1% O
2
). b, EMSA using the Epo
HRE. N, normoxia; H, hypoxia (1% O
2
, 4 h). In HeLa and RCC4/VHL, HIF-1 is a
doublet (S, slower and F, faster mobility). RCC4 cells contain only faster mobility
HIF-1, with equivalent levels in normoxia and hypoxia. Constitutive binding
species are indicated (C). c, Immunoblots of whole cell extracts for HIF a-
subunits. Upper panels; RCC4 and RCC4/VHL cells (+VHL). Lower panel; 786-O
cells stably transfected with vector alone, a full length VHL gene (+VHL), or a
truncated VHL gene (1-115; +Tr). HIF-1awas not detected in 786-O cells. d,
Immunoblot of UMRC2, UMRC3 and KTCL140 (renal carcinoma lines with VHL
mutations30), Caki-1 (renal carcinoma line expressing wild-type pVHL) and the
hepatoma line Hep3B.
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by anti-HIF-1a(Fig. 3e). Similar results were obtained in other cell
lines (Hep3B, Caki-1, MRC5-V2 and 293; data not shown).
Furthermore, whereas RCC4/VHL, HeLa and other cells contained
both HIF-1 species, RCC4 extracts contained only the faster-
mobility species (Figs 3d, Fig. 2b). Thus, VHL-defective cells lack
the slower-mobility species which is restored by re-expression of
pVHL, and shifted by anti-pVHL. This indicates that the DNA-
binding HIF-1 doublet arises from two species containing or not
containing pVHL. Combination supershift analysis confirmed that
the slower-mobility species contained both HIF-1aand pVHL
(Fig. 3e).
HIF-1 activation by hypoxia is mimicked by cobaltous ions and
iron chelation6,7. We therefore tested whether the pVHL/HIF-1
interaction was regulated by these stimuli. Proteasomal blockade
induces an HIF-1 DNA-binding complex in normoxic cells18;
comparison of this normoxic complex with EMSA of hypoxic
cells with or without proteasomal inhibitors showed a similar
shift and anti-pVHL supershift (Fig. 4a and data not shown).
Together with immunoprecipitation data, this indicates that the
interaction with pVHL occurs in both normoxic and hypoxic cells.
In contrast, EMSA analysis of RCC4/VHL cells treated with cobalt
and the iron chelator desferrioxamine (DFO) demonstrated only
the faster mobility HIF-1. This did not supershift with anti-pVHL,
indicating that the pVHL/HIF-1 complex could not form in cells
exposed to these stimuli. Similar results were obtained in other cell
types and are consistent with hitherto unexplained mobility differ-
ences between previous analyses of HIF-1 from cobalt- or DFO-
versus hypoxia-stimulated cells7, indicating that this is a general
effect. Treatment with DFO 4 h before hypoxia prevented the
formation of the pVHL/HIF-1 complex (Fig. 4b). Addition of
iron chelators could not break the pVHL/HIF-1 complex in vitro,
whereas addition of in vitro-translated wild-type pVHL (but not a
truncated pVHL) could restore the slower-mobility species to
nuclear extracts of proteasomally blocked, normoxic and hypoxic
RCC4 cells, but not to cells treated with DFO or cobalt (Fig. 4c, and
Fig. S1a of Supplementary Information). Immunoprecipitation
studies also indicated that the interaction between HIF-1 and
pVHL is iron-dependent. Whereas both HIF-1aand HIF-2awere
contained in anti-pVHL immunoprecipitates from hypoxic RCC4/
VHL cells, neither was contained in precipitates from DFO- or
cobalt-treated cells (Fig. 4d). The iron-dependent interaction
between HIF a-subunits and pVHL may be direct or indirect.
However, in vitro-translated wild-type pVHL did not bind to an
in vitro-translated HIF-1 DNA-binding complex (Fig. S1b in
Supplementary Information), in contrast to the interaction with
RCC4 extracts (Fig. 4c), indicating that an additional factor or
modification of HIF-1 not represented in rabbit reticulocyte lysates
is necessary for the association.
Normally, HIF a-subunits are targeted for rapid degradation in
normoxic cells by a proteasomal mechanism operating on an
internal oxygen-dependent-degradation (ODD) domain5. Our
data suggest that pVHL might be required for this process—a
possibility which would be consistent with recent data that pVHL
forms a multiprotein complex (containing Cul-2 and elongins B
and C) which has homology with ubiquitin-ligase/proteasome-
targeting complexes in yeast19,20. When cells were switched from
Figure 3 Association of pVHL with HIF-1. a, Immunoblots for HIF a-subunits (2a,
1a) of IG32 (VHL ip) and control immunoprecipitates (using VG-7be) of RCC4/VHL
(VHL+) and RCC4 (VHL) cells exposed (4 h) to normoxia or hypoxia (1% O
2
; H+)
with or without proteasomal inhibition (PI+). Aliquots of selected input lysates
were also immunoblotted. b, Immunoprecipitation of RCC4/VHL (VHL+) and
RCC4 (VHL) extracts with polyclonal antibodies to HIF a-subunits or normal
rabbit immunoglobulin (control ip) followed by immunoblotting for pVHL (V). A
cross-reacting species arose from the HIF-2aantibody (asterisk). c, Immuno-
precipitation of HeLa extracts with IG32 (VHL
ip
) or pAb419 (control) followed by
immunoblotting for HIF a-subunits. d, Anti-pVHL supershifts. IG32 (VHL Ab+) or
VG-7be (Control Ab+) was added to binding reactions of nuclear extracts from
normoxic or hypoxic (1% O
2
, 4 h; H+) cells. Anti-pVHL supershifted (SS) the slower
HIF-1 species (S) in HeLa and hypoxic RCC4/VHL cells. No supershift was seen
with RCC4 cells, which lack pVHL. e, Anti-pVHL, anti-HIF-1aand combination
supershifts in HeLa cells. Anti-pVHL (IG32, +VHL Ab) supershifted slower mobility
HIF-1. Anti-HIF-1a(clone 54) supershifted both components (SS). Addition of both
antibodies ‘super-super-shifted’ (SSS) slower mobility HIF-1.
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hypoxia to normoxia with addition of cycloheximide, HIF a-
subunits decayed with a half-life of about 5 min in wild-type VHL
transfectants, compared with ,60 min in the VHL-defective RCC4
and 786-O cells, thus confirming a strong effect of pVHL on stability
(Fig. 5a). Moreover, functional studies of Gal4 chimaeras contain-
ing the HIF-1aODD domain demonstrated a striking dependence
of the isolated ODD domain on pVHL (Fig. 5b).
These experiments define a function for pVHL in the regulation
of HIF-1. Given recent demonstrations of the importance of HIF-1
in tumour angiogenesis3,21, constitutive HIF-1 activation is clearly
consistent with the angiogenic phenotype of VHL disease. Whether
it is a sufficient explanation for oncogenesis is less clear. HIF-1
mediates gene activation not only by hypoxia, but also by growth
factors such as insulin and insulin-like growth factor-1 (ref. 22).
HIF-1 targets such as molecules involved in enhanced glucose
metabolism and angiogenesis8,9,14 are classically upregulated (by
different mechanisms) in many forms of cancer, supporting an
important role in tumour progression, although their role in the
initiation of oncogenesis is less clear. pVHL is probably a multi-
functional protein which could have other tumour suppressor
actions11,23,24. One possibility is that other gene products could be
targeted in a similar manner to HIF a-subunits; however, compari-
son of anti-pVHL immunoprecipitates from metabolically labelled
RCC4/VHL cells with and without proteasomal inhibitors has so far
demonstrated only two species: HIF-1aand HIF-2a(G.-W.C.,
unpublished observations).
As both pVHL and HIF-1 are widely expressed, it is likely that the
physiological role of pVHL in HIF-1 regulation is general. It is not
yet clear whether pVHL has actions on oxygen-regulated gene
expression other than through HIF-1. Stabilization of hypoxia-
inducible mRNAs has been reported in VHL-deficient cells12,13. This
might represent an independent action of pVHL. However, regula-
tion of these RNAs is commonly abolished in HIF-1-deficient
cells3,25, so the mRNA stability factors could lie downstream of
Figure 5 Effect of pVHL on HIFastability and ODD domain function. a, Western
analysis of HIF a-subunit stability in cells lacking VHL (RCC4, 786-O) and stable
transfectants expressing pVHL (+VHL). Cells were incubated in hypoxia (4 h),
then moved to normoxia (time 0) with addition of cycloheximide (100 mM) and
harvested up to 80 min later. b, Representative functional assay of the HIF-1aODD
domain. Hep3B or RCC4 cells were transfected with Gal4 reporter pUAS-tk-Luc,
and either pGalVP16 (upper panel) encoding the Gal4 DNA-binding domain fused
to the VP16 activation domain, or pGala344-698VP16 (lower panel) which includes
HIF-1aamino acids 344– 698 (containing the entire ODD domain5). RCC4 cells
were co-transfected with pcDNA3 (), pcDNA3-VHL (VHL) or pcDNA3-VHL.103FS
(TrVHL). After transfection, cells were divided for 24 h incubation in normoxia (N)
or hypoxia (H; 0.1% O
2
). Corrected luciferase counts are shown, normalized to the
normoxic value with pGalVP16 or pGalVP16+pcDNA3. The HIF-1adomain con-
fers suppression and hypoxic regulationin Hep3B cells but not RCC4 cells, where
re-expression of pVHL restores these properties.
Figure 4 Effect of cobaltous ions and iron chelation on the pVHL/HIF-1
interaction. a, EMSA and supershift analysis of RCC4/VHL cells exposed (4 h)
to normoxia (N), hypoxia (H; 1% O
2
), DFO (100 mM), cobaltous chloride (Co,
100 mM) or proteasomal inhibition (PI). In lanes 6– 10, IG32 was added (+aVHL). b,
EMSA and supershift analysis of RCC4/VHL cells subjected to hypoxia (H; 1% O
2
for 8 h) or DFO (100 mM) with hypoxia (D H; cel ls exposed to DFO for 8 h, 4 h
normoxia, then 4 h hypoxia). c, EMSA of RCC4 cells showing only faster mobility
HIF-1 (lanes 1– 4). Addition of in vitro transcribed/translated pVHL alone (lanes 5–
8, +IVTT), or together with IG32 (lanes 9– 12, +IVTT+aVHL); slower mobility HIF-1
forms and supershifts in cells exposed to normoxia, hypoxia and PI, but not DFO.
d, HIFaimmunoblots of anti-pVHL immunoprecipitates of RCC4/VHL cells.
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HIF-1. Positive and negative effects on HIF-1 activation by iron
chelators, hydrogen peroxide and redox active agents have indicated
that the underlying oxygen sensing mechanism may involve oxygen-
dependent generation of partially reduced oxygen radical species by
redox active iron centre(s)2,6,7,15,26. Such system(s) could act distantly
on a transduction pathway or might directly modify the ODD
domains of HIFathrough a caged radical system, as proposed for
iron sensing in the regulation of IRP2 (ref. 27). Cobaltous ions
activate HIF-1 in inverse relation to iron availability, indicating that
there may be competition for incorporation into a metal centre28.
Our data support a model in which VHL/HIF complexes form in
normoxic cells and target HIFasubunits for destruction. In
hypoxia, degradation is suppressed despite complex formation,
perhaps because a critical targeting modification of the HIFa
ODD domain cannot occur without oxygen. Cobaltous ions and
desferrioxamine prevent formation of the VHL complex, providing
a different mechanism for stabilization of HIF-1 and potentially
explaining why activation by these stimuli is relatively resistant to
oxygen and certain other radical-generating processes6,16,29. The iron
dependence of complex formation could indicate that an iron-
containing protein is an essential component of the complex,
perhaps involved in local generation of an oxygen-sensing signal. M
.........................................................................................................................
Methods
Cells and transfections. 786-O cells expressing pVHL, truncated pVHL
(amino acids 1– 115), or empty vector17 were a gift from W. G. Kaelin. RCC4
cells were a gift from C. H. C. M. Buys. Other RCC lines were provided by M.
Lerman. HeLa and Hep3B cells were fromECACC. RCC4/VHL was obtained by
transfection with pcDNA3-VHL and G418 selection. Cells were plated in
medium lacking G418 24 h before experiments, which were performed on
75 cm
2
dishes approaching confluence. Proteasomal inhibition was with
100 mM calpain inhibitor I and 10 mMN-carbobenzoxyl-L-leucinyl-L-norval-
inal. Transient transfections were by electroporation. Transfected cells were
split for parallel normoxic and hypoxic incubation (Napco 7001, Precision
Scientific). Luciferase reporter gene activity was corrected for transfection
efficiency by assay of b-galactosidase expression from the co-transfected
control plasmid pCMV–bGal.
RNA analysis. Total RNAwas extractedand analysed by ribonuclease protection.
Riboprobe details are given in Table S1 of the Supplementary Information.
Plasmid constructions. pCDNA3-VHL contained nucleotides 214–855 of
GenBank accession no. L15409in pcDNA3 (Invitrogen). pcDNA3-VHL.103FS
was made using site-directed mutagenesis to delete nucleotides 522–523. HRE
reporter genes were based on pGL3-basic (Promega) or pPUR (Clontech) and
contained either a minimal SV40 promoter or a minimal (40 bp) thymidine
kinase promoter linked to a firefly luciferase gene (details of HREs are in
Supplementary Information S3). pGalVP16 encoded the Gal4 DNA-binding
domain (amino acids 1– 147) linked in-frame to the activation domain (amino
acids 410–490) from herpes simplex virus protein 16; pGala344–698VP16
encoded the indicated amino acids of HIF-1abetween those domains. Plasmid
pUAS-tk-Luc contained two copies of the Gal4 binding site linked to a
thymidine-kinase-promoted luciferase reporter gene.
Cell lysis, immunoblotting and immunoprecipitation. Whole cell extracts
were prepared by homogenization in denaturing conditions and aliquots
immunoblotted for HIF a-subunits with 28b (anti-HIF-1a) and 190b (anti-
HIF-2a) as described16, or using clone 54 (anti-HIF-1a, Transduction Labora-
tories). For immunoprecipitation, lysis was performed in 100 mM NaCl, 0.5%
Igepal CA630, 20 mM Tris-HCl (pH 7.6), 5mM MgCl
2
and 1 mM sodium
orthovanadate with aprotinin (10 mgml
1
), ‘Complete’ protease inhibitor
(Boehringer) and 1.0 mM 4-(2-aminoethyl)benzene sulphonyl fluoride for
30 min on ice. After clearance by centrifugation, 120mg aliquots of lysate
were incubated for 2 h at 4 8C with 4 mg affinity-purified anti-HIF-2apoly-
clonal antibodies (raised against a bacterially expressed fusion protein includ-
ing amino acids 535– 631) or 4mg ammonium sulphate precipitated anti-HIF-
1apolyclonal antibodies (raised against an immunogen including amino acids
530–652) in parallel with normal rabbit immunoglobulin (control), or alter-
natively with 0.7 mg anti-pVHL antibody (IG32, Pharmingen) or control
(antibody to SV40 T antigen, pAb419, a gift from E. Harlow or antibody to
VEGF, VG-7be, a gift from H. Turley). 10 ml conjugated agarose beads pre-
blocked with 20 mg ml
1
BSA was added and lysates incubated for 2 h with
rocking. Pellets were washed five times, eluted with sample buffer, and divided
into 2– 6 aliquots for immunoblotting.
Electrophoretic mobility shift and supershift assays. We prepared nuclear
extracts using a modified Dignam protocol and incubated 5mg (HeLa) or
7.5 mg (RCC4) with a
32
P-labelled 24-bp oligonucleotide probe (sense strand;
59-GCCCTACGTGCTGCCTCGCATGGC-39) from the mouse Epo 39enhan-
cer as described25. For supershift assays, 0.5 mg IG32, VG-7be (isotype and
subclass matched control for IG32) or clone 54 (anti-HIF-1a) was added and
reactions were incubated for 4 h at 4 8C before electrophoresis. In vitro
transcription translations of pcDNA3-VHL and pcDNA3-VHL.103FS were
done using reticulocyte lysate (Promega); 1 ml of a 1:5 dilution in PBS was
added to binding reactions 2 h before electrophoresis or addition of antibody.
Received 11 March; accepted 13 April 1999.
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as paper copy from the London editorial office of Nature.
Acknowledgements. We thank W. Kaelin, C. Buys and M. Lerman for cell lines, and N. Proudfoot,
A. Harris, D. Gillespie, J. O’Rourke,Y.-M. Tian and L. Nicholls. Financial support was from the Wellcome
Trust, the Barnes Trust, the Deutsche Forschungsgemeinschaft, the Cancer Research Campaign, Action
Research and the Medical Research Council.
Correspondence and requests for materials should be addressedto P.J.R. (e-mail: peter.ratcliffe@imm.ox.
ac.uk).
... 3 Normoxia, a family of iron-and 2-oxoglutarate-dependent dioxygenase HIF prolyl hydroxylases (HIF-PHDs, HIF-PHD1−3), 4 can hydroxylate certain prolines of HIF-α leading to the recognition of HIF-α by Von Hippel− Lindau protein (pVHL) and subsequent degradation via the ubiquitin-proteasome system. 5,6 Concurrently, factor inhibiting hypoxia-inducing factor (FIH), an asparaginyl hydroxylase, can prevent HIF-α from associating with the coactivator p300/CBP and thus results in a reduction of HIF-mediated transcription 7 ( Figure 1A). Among these HIF-related hydroxylases, HIF-PHD2 probably plays the dominant role in controlling HIF-α-mediated endogenous EPO expression. ...
... The inhibition activities against HIF-PHD2 of all the click products were tested by fluorescence polarization (FP) assay. 19 Triazole compound 6e, which was clicked by 4e and benzyl azide (5), reached the best activity of 853.8 nM. Subsequently, we fixed "head" fragment 4e and screened a series of azide "tail" fragments 12a−o, which included electronwithdrawing substituted benzyl azides, electron-donating substituted benzyl azides, and polysubstituted benzyl azides. ...
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Hypoxia results in adaptive changes in the transcription of a range of genes including erythropoietin. An important mediator is hypoxia-inducible factor-1 (HIF-1), a DNA binding complex shown to contain at least two basic helix-loop-helix PAS-domain (bHLH-PAS) proteins, HIF-1 alpha and aryl hydrocarbon nuclear receptor translocator (ARNT), In response to hypoxia, HIF-1 alpha is activated and accumulates rapidly in the cell. Endothelial PAS domain protein 1 (EPAS-1) is a recently identified bHLH-PAS protein with 48% identity to HIF-1 alpha, raising the question of its role in responses to hypoxia. We developed specific antibodies and studied expression and regulation of EPAS-1 mRNA and protein across a range of human cell lines. EPAS-1 was widely expressed, and strongly induced by hypoxia at the level of protein but not mRNA. Comparison of the effect of a range of activating and inhibitory stimuli showed striking similarities in the EPAS-1 and HIF-1 alpha responses. Although major differences were observed in the abundance of EPAS-1 and HIF-1 alpha in different cell types, differences in the inducible response were subtle with EPAS-1 protein being slightly more evident in normoxic and mildly hypoxic cells. Functional studies in a mutant cell line (Ka13) expressing neither HIF-1 alpha nor EPAS-1 confirmed that both proteins interact with hypoxically responsive targets, but suggest target specificity with greater EPAS-1 transactivation (relative to HIF-1 alpha transactivation) of the VEGF promoter than the LDH-A promoter. (C) 1998 by The American Society of Hematology.
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The hypoxia-inducible factor 1 transcriptional activator complex (HIF-1) is involved in the activation of the erythropoietin and several other hypoxia-responsive genes. The HIF-1 complex is composed of two protein subunits: HIF-1β/ARNT (aryl hydrocarbon receptor nuclear translocator), which is constitutively expressed, and HIF-1α, which is not present in normal cells but induced under hypoxic conditions. The HIF-1α subunit is continuously synthesized and degraded under normoxic conditions, while it accumulates rapidly following exposure to low oxygen tensions. The involvement of the ubiquitin-proteasome system in the proteolytic destruction of HIF-1 in normoxia was studied by the use of specific inhibitors of the proteasome system. Lactacystin and MG-132 were found to protect the degradation of the HIF-1 complex in cells transferred from hypoxia to normoxia. The same inhibitors were able to induce HIF-1 complex formation when added to normoxic cells. Final confirmation of the involvement of the ubiquitin-proteasome system in the regulated degradation of HIF-1α was obtained by the use ofts20TG R cells, which contain a temperature-sensitive mutant of E1, the ubiquitin-activating enzyme. Exposure of ts20 cells, under normoxic conditions, to the non-permissive temperature induced a rapid and progressive accumulation of HIF-1. The effect of proteasome inhibitors on the normoxic induction of HIF-1 binding activity was mimicked by the thiol reducing agentN-(2-mercaptopropionyl)-glycine and by the oxygen radical scavenger 2-acetamidoacrylic acid. Furthermore,N-(2-mercaptopropionyl)-glycine induced gene expression as measured by the stimulation of a HIF-1-luciferase expression vector and by the induction of erythropoietin mRNA in normoxic Hep 3B cells. These last findings strongly suggest that the hypoxia induced changes in HIF-1α stability and subsequent gene activation are mediated by redox-induced changes.
Article
Hypoxia-inducible expression has been demonstrated for many groups of mammalian genes, and studies of transcriptional control have revealed the existence of hypoxia-responsive elements (HREs) in the cis-acting sequences of several of these genes. These sequences generally contain one or more binding sites for a heterodimeric DNA binding complex termed hypoxia-inducible factor-1 (HIF-1). To analyze this response further, Chinese hamster ovary cells were stably transfected with plasmids bearing HREs linked to genes encoding immunoselectable cell surface markers, and clones that showed reduced or absent hypoxia-inducible marker expression were selected from a mutagenized culture of cells. Analysis of these cells revealed several clones with transacting defects in HRE activation, and in one the defect was identified as a failure to express the α-subunit of HIF-1. Comparison of hypoxia-inducible gene expression in wild type, HIF-1α-defective, and HIF-1α-complemented cells revealed two types of response. For some genes (e.g. glucose transporter-1), hypoxia-inducible expression was critically dependent on HIF-1α, whereas for other genes (e.g. heme oxygenase-1) hypoxia-inducible expression appeared largely independent of the expression of HIF-1α. These experiments show the utility of mutagenesis and selection of mutant cells in the analysis of mammalian transcriptional responses to hypoxia and demonstrate the operation of HIF-1α-dependent and HIF-1α-independent pathways of hypoxia-inducible gene expression in Chinese hamster ovary cells.